1. Field of the Invention
The present invention relates to a step of focusing a laser beam to the inside of the semiconductor wafer and an adhesive sheet used for a step of separating a semiconductor wafer by expanding the adhesive sheet. Also, the present invention relates to a production method of the semiconductor wafer device using said adhesive sheet.
2. Description of the Related Art
After a circuit is formed on a surface of the semiconductor wafer, a backside grinding step which grinds the backside of the wafer and control the thickness of the wafer is performed, and a dicing step which separates the wafer into a predetermined chip size is performed.
Recently, along with the wide spread of the IC card, the semiconductor chip which is the constituting member of the IC card, has become thinner. Thus, although the thickness was conventionally 350 μm or so, the semiconductor wafer is demanded to be as thin as 50 to 100 μm or even less.
As the wafer is a brittle member, the risk of being broken during the transportation or so increases as it gets thinner. When such ultra thin wafer is cut by a dicing blade which rotates in a high speed, the chipping or so are produced particularly on the backside of the semiconductor wafer; and significantly lowers the transverse strength of the chip.
Thus, so called a stealth dicing method is proposed (Patent document 1) which irradiates the laser beam to the inside of the semiconductor wafer; thereby forming a dicing line while selectively forming the modified portion, then cuts the semiconductor wafer by using the modified portion as the starting point. According to the stealth dicing method, after forming the modified portion by irradiating the laser beam to the inside of the semiconductor wafer, the ultra thin semiconductor wafer is adhered to the adhesive sheet comprising the substrate and the adhesive layer, then by expanding the adhesive sheet, the semiconductor wafer is divided along the dicing line (dicing), thereby the semiconductor chip can be produced efficiently.
However, in such stealth dicing method, the adhesive sheet (the dicing sheet) used for the conventional dicing step was used when producing a semiconductor chip having a very small size. When this is used, the stress (the expanding force) during the expanding of the adhesive sheet is adsorbed by the adhesive sheet, thus the expanding force cannot sufficiently transferred to the modified portion inside the wafer. Hence it was difficult for the wafer to efficiently form the chip. When the expanding force is not sufficiently transferred, each chip is not divided at the dicing line, which causes to connect plurality of the chips and has a risk to lower the production efficiency of the semiconductor chip. Also, even in case the substrate side of the adhesive sheet is scratched by the jigs or so simultaneously as the expanding, it was still difficult to efficiently produce the small chips.    [Patent Document 1] Japanese Patent No. 3762409